If I remember correctly ZFS keeps the whole array running whenever one is active (which is basically always). If I remember, I'll check my UPS when I get home to see the actual power draw. The storage itself is probably cheaper to run than the main server in the rack - a gen8 HP 360p, which is a bit on the old side and I'd guess not terribly efficient being a 1U piece with many small high-powered fans running constantly.

Electricity here isn't too expensive though, being public hydro power.